Polycarbonates are a particularly useful class of polymers because they have exceptionally high impact resistance and ductility. Also, polycarbonates can be made with high optical clarity. Some common uses of polycarbonates include transparent window glazing and optical media applications (e.g., the CD, DVD, CD-RW and other formats).
The present invention relates to particular polycarbonates, efficient processes for manufacturing and distributing polycarbonates, articles made from polycarbonates, and methods for making polycarbonate articles.
Polycarbonates are desirable materials for a wide variety of end use applications including window glazing, optical media, automotive parts, building materials, packaging, and many others. Each of these end use applications has slightly different requirements, and it is desirable to customize the properties of polycarbonates to match the differing demands of particular end use applications. For example, it is desirable to use polycarbonate having a relatively low melt flow index (i.e., high viscosity) to make polycarbonate sheet for glazing applications because polycarbonate sheet is most economically produced by extrusion operations which are not compatible with low viscosity materials. In contrast, it is better to use high melt flow index (i.e., low viscosity) polycarbonate to make complicated parts by injection molding because it is easier to ensure that a lower viscosity material will completely fill a mold cavity having a complicated shape with small openings.
To meet requirements of diverse applications, manufacturers have typically run alternating manufacturing xe2x80x9ccampaignsxe2x80x9d of polycarbonate resins. Specifically, manufacturers typically produce a large selection of polycarbonates having differing melt flow indices by varying polymerization conditions (e.g., time, temperature, etc.) to produce materials having differing average molecular weights. This process is expensive because it requires delay when switching between melt flow index (hereinafter xe2x80x9cMFIxe2x80x9d) set points, and in continuous processes produces large quantities of off-specification material during the switch. There has long been a desire to eliminate this costly switching step while maintaining the flexibility to deliver a variety of customized polycarbonate grades.
It is known that certain xe2x80x9credistribution catalystsxe2x80x9d may be used to promote chemical reactions which cut apart polycarbonate molecules. Examples of suitable redistribution catalysts are described, for example, in U.S. Pat. Nos. 5,567,802, 5,886,073, 5,459,226, and 5,141,057. These catalysts offer the ability to reduce the average molecular weight of polycarbonate, thereby raising the average MFI of the polycarbonate. However, certain of these catalysts may be too active to allow fine control of MFI. Also, the mere addition of a fixed amount of a redistribution catalyst to a source stream of polycarbonate would not necessarily make the MFI more homogenous throughout a large campaign because prior to addition of the catalyst, the stream of polycarbonate may have substantial MFI variation.
Further difficulties in manufacturing commercially useful polycarbonate products are encountered when attempting to produce large quantities of polycarbonate having both a consistent average MFI and a narrow distribution of MFI. In conventional manufacturing operations, large quantities of polycarbonate may be blended to obtain a homogeneous batch where the average MFI does not vary greatly. However, this technique is cumbersome, expensive and may still result in a product having a wide distribution of MFI. This wide MFI distribution is undesirable because it produces variability which causes problems with long term consistency when processing the resin into finished articles. For example, resins with wide MFI distributions are more likely to cause errors when injection molding because the plasticizing time of the resin is less predictable. Also, the peak injection pressure variability in injection molding operations may be higher than expected for such resins. Alternatively, the manufacturer may simply ship polycarbonate having a wider specification. This solution is also undesirable because drift of the mean MFI within a wide specification may require constant readjustment of injection molding and other fabrication operations. The assignee of the present application has endeavored to make higher quality polycarbonate by reducing MFI variability in its commercial polycarbonate products, allowing more precise tuning and less adjustment of molding parameters.
The present invention provides a method for producing a redistributed resin having a narrowed molecular weight distribution. According to this method, a source polycarbonate resin is fed into an extruder together with a redistribution catalyst for said resin, and a feedback loop control mechanism is used to adjust a result effective parameter (i.e., a parameter that will effect the MFI of the redistributed resin). Typical result effective parameters include the feeding rate of a redistribution catalyst, the temperature of the extruder, the operation speed of the extruder, the feeding rate of a co-reactant, the feeding rate of a quenching agent, the feeding ratio of the redistribution catalyst and combinations thereof.
The present invention also provides a method for producing a redistributed thermoplastic resin which comprises feeding a resin and a redistribution catalyst for said resin into an extruder, measuring the current melt temperature, comparing the melt temperature with a set point melt temperature to determine a temperature differential, and adjusting a result effective parameter in response to the temperature differential. It is further possible to instead measure the viscosity, compare the viscosity with a set point viscosity to determine a viscosity differential, and adjust the result effective parameter in response to said viscosity differential. It is also possible to measure both the temperature and viscosity, and to set the set point temperature or the set point viscosity based on the viscosity differential or the temperature differential, respectively.
In another aspect, the invention relates to a redistributed polycarbonate resin produced by the process described herein.
In yet another aspect, the invention relates to an improved method for injection molding which comprises injection molding a redistributed polycarbonate resin produced by the process described herein.
The present invention further relates to optical media, such as CD""s, DVD""s, CD-RW""s and future commercial formats which comprise a redistributed polycarbonate resin produced by the process described herein.
The invention further relates to a method for producing a redistributed resin having a narrowed MFI distribution, which method comprises extruding a polycarbonate source resin together with a redistribution catalyst for said resin while using a feedback loop control mechanism to adjust addition of said redistribution catalyst.
In another aspect, the invention relates to a method for minimizing the amount of off-specification polycarbonate produced by a polycarbonate plant. According to this method, a master grade of polycarbonate is produced at a manufacturing facility. The master grade is then transported to a redistribution facility which may or may not be in a different geographic location. At the redistribution facility, the master grade is extruded together with a redistribution catalyst.